Tsunamis and Seiches

Key Points

Tsunami

Definition: Tsunamis are water waves caused by earthquakes, volcanic eruptions, or landslides. In deep water tsunamis have long wavelengths, short wave heights, and travel up to 30 mph. As tsunamis enter shallow water they slow down, and the waves increase significantly in height.

Tsunamis do not have to have large wave heights to be damaging. Tsunami damage is caused by both the forces exerted by flowing water onto structures, and by run-up of the wave onto land that causes flooding and carries debris. Tsunamis can also generate dangerous current speeds that can be hard for vessels to navigate.

Tsunamis generated in the Pacific Ocean off Washington's coast will not have as great of an effect in Seattle as they will on the Pacific Coast, but low-lying areas may experience flooding, and strong currents will likely be present in Puget Sound for hours after the earthquake.

Tsunamis can be generated in Puget Sound by both landslides and earthquakes.

The most frequent cause of Puget Sound tsunamis is landslides. The 1949 Olympia earthquake triggered a landslide in the Tacoma Narrows that caused a 6 to 8-foot tsunami that affected nearby shorelines three days after the earthquake.

The most damaging tsunami would likely come from a Seattle Fault earthquake, or earthquakes on other local faults. There is evidence that an earthquake on the Seattle Fault that occurred around 900 AD produced a 16-foot tsunami. The National Oceanic and Atmospheric Administration (NOAA) recreated this tsunami using a model.

The modelled tsunami would flood areas up to one mile inland with depths up to 5 meters. The tsunami would hit immediately after the ground stopped moving. People along the shore would have little time to escape. It would destroy buildings along the shore and flood low-lying areas up to one mile inland. Structures built to modern code would fare better than older ones.

The 900 AD tsunami was probably a worst-case event. It is more likely (but not certain) that the next Seattle Fault earthquake and tsunami will be smaller.

The preliminary results from modeling a tsunami generated by an M9.0 megathrust earthquake predict the main impacts to be current speeds of 3 to 5 knots in Elliott Bay and 15 inches of inundation near the Duwamish river.

Seiche

Definition: Seiches are standing waves in waterbodies caused by most often by seismic waves or atmospheric pressure. They can occur at great distances (100s or 1000s of miles away) from an earthquake epicenter. Because they are standing waves they move vertically rather than horizontally.

Lake Union is especially prone to seiches due to its shape. The east and west sides are roughly parallel, and the V-shaped northern end focuses waves. There is a historical report of a seiche or tsunami on Lake Washington, but it is not clear how large seiches on Lake Washington could be.

Seiches have occurred multiple times in Seattle, but they have not caused extensive damage so far. Large seiches are a danger to the I-90 and SR 520 floating bridges. A large seiche could strain cables anchoring the bridges. The new SR 520 bridge is designed to take about 12-feet of upward motion and 8-feet of downward motion from a seiche. Based on models the most damaging seiche would probably be caused by a Cascadia subduction zone earthquake.